A conventional coated fine metallic wire used as a bonding wire in a semiconductor device is composed of a fine metallic wire and an insulative layer formed therearound, and its exterior view is the same as that of a coated fine metallic wire according to the invention shown in FIG.1.
As an example of the aforementioned coated fine metallic wire, that insulated by heat resisting epoxy resin is described in Japanese patent Kokai 7-268278. Moreover, in the above mentioned reference, polyurethane resin, nylon resin, polyimide resin, resin of fluorine series, polyethylene resin, poly vinyl chloride resin, enamel as varnish, polyester resin and heat resisting polyurethane resin are described as known material used for the same purpose.
Among the coated fine metallic wires covered with aforementioned known material, that covered with polyester is unsuited for bonding and failure in bonding is apprehended, for rigidity of polyester is too high. Heat resisting properties of polyester resin, polyimide resin, nylon resin and resin of fluorine series are high. However, each of these is not resolved, but carbonized by heating temperature at the time of bonding, especially in case that an end portion of the coated fine metallic wire is melted and formed into a metallic ball. Accordingly, a carbonized matter stuck to the coated wire or the metallic ball is caught by a capillary, and disturbs supply of the wire and bonding between the metallic ball and an electrode terminal. On the other hand, in order to bond the coated wire with a lead with high reliability, it is necessary to thermally remove the insulative layer at a bonding part of the coated wire on the side of the lead by means of discharge process, but a carbonized matter remains on the surface of the metallic wire, hence subsequent bonding cannot be carried out securing high reliability.
In heat resisting epoxy resin used as material of the insulative layer, uplift of heat resisting property, avoidance of occurrence of carbonized matter and improvement of reliability of bonding are intended. However, even in case of the coated fine metallic wire with the insulative layer formed of heat resisting epoxy resin, removal of the insulative layer is carried out by means of discharge process, so that it is difficult to remove the insulative layer without melting the fine metallic wire and securing stability.